(Invited) Potential and Activities of III-V/Si Tandem Solar Cells

III-V compound/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. However, problems to be solved are high density of dislocations and large residual strain in III-V compound on Si in the case of hetero epitaxy of III-V compound materials on Si substrates. There are other approaches such as epitaxial lift-off and direct bonding technologies in order to prevent such problems.    This paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large misfit and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3x10⁵ cm^-2 is necessary.    The paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells. Especially, effective dislocation density reduction in III-V compound films on Si substrates by thermal cycle annealing (TCA) and insertion of strained-layer super-lattices (SLS) in our study are demonstrated. As a result of combination of TCA process and SLS insertion, high quality GaAs films with a dislocation density of 1-2x10⁶ cm^-2 have been realized.  Reduction in dislocation density of GaAs-on-Si layers due to TCA and SLS insertion have been explained by coalescence of dislocations under high temperature and high stress condition and dislocation movement towards edge and interface by mismatch stress, respectively. In addition, effectiveness of thicker-layer growth, selective area growth, low-temperature growth for hetero epitaxy of III-V compound films on Si substrates are also reviewed.

   We have demonstrated 20% efficiency (under 1-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si. By employing high quality GaAs films on Si, heteroface GaAs thin films solar cells with an Al_0.8Ga_o.2As window layer were fabricated on Si substrates. The thickness of Si substrates was 200mm. The n-GaAs base layer (carrier concentration of 5x10¹⁷ cm^-3) with a thickness of 1.3-1.5mm was grown on an Al_0.6Ga_0.4As/GaAs super lattice (SL) layer (20nm/100nm x 5pairs), In_0.1Ga_0.9 As/GaAs SLS (10nm/10nm x 5 pairs), and thermal cycle annealed GaAs buffer layer (TCA:900⁰C x 5 times, 2mm thick) which were grown on Si substrates. The thickness and carrier concentration of p-GaAs emitter layer were 0.5 mm and 2x10¹⁸ cm^-3, respectively. The total epitaxial layer thickness was about 3.5-5 mm. A Si₃N₄ layer was used as an anti-reflection film. Ti/Ag was used as electrode metal. The cell area was 1 cm². The total-area conversion efficiency of 18.3% at AM0 and 20.0% at AM1.5G were the highest ever reported for GaAs-on-Si single-junction solar cells.  Such high-efficiency GaAs-on-Si cells have been attained by combining TCA, SLS and SL buffer layers. As a result of such a combination, high efficiency GaAs-on-Si solar cells have been realized using high quality GaAs films with a dislocation density of 3.7x10⁶ cm^-2. The first space flight of forty-eight 2 cm x 2 cm GaAs-on-Si solar cells with an average AM0 total-area efficiency of 16.9% have been carried out using the Engineering Test Satellite No. 6 (ETS-VI). The GaAs-on-Si solar cells have been demonstrated to be more radiation-resistant on space than GaAs-on-GaAs solar cells and 50, 100 and 200-mm-thick Si space solar cells. These results show that the III-V/Si-based multi-junction solar cells have great potential for space applications.    Ohio State University has also demonstrated 17.1% (AM0) efficiency with GaAs single-junction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding.

   Future prospects are also discussed. Further improvement in efficiency for GaAs-on-Si single-junction solar cells and III-V/Si multi-junction solar cells can be obtained by reducing dislocation densities to less than 3x10⁵ cm^-2.